Do you mean it can send a pre-encoded AC3 stream over S/PDIF, or that it can actually encode AC3 in realtime? The latter would be pretty impressive, but I doubt this is a "factory" feature.

You may wish to start by looking at http://opensource.creative.com. There is enough information there to get you outputting AC3 files that you have on hand...search the web if you don't have any, or get a copy of SoundForge's encoder. Or rip one off a DVD.

You could try getting a developer account at Creative if you wish to use the Windows API, but in my experience this is not possible. I faxed the NDA in three times and they never gave me an account or responded to my emails.

Good luck...I'd be thrilled if you came up with something. It's a difficult task but a very worthy one, IMHO. Besides, it lays a lot of groundwork for a DIY decoder. :)

Dolby charges a horrendous amount to use AC-3 technology, so creative didn't want to front the bucks. Instead the card is designed to output multiple signals of plain S/PDIF. 3 separate signals (and cables) for 5.1.

This raises an interesting question: Can a home theatre reciever handle 3 different digital pcm streams over optical or is it no different then coax, AC-3 or multiple lines?

See, I have an optical input on my reciever, and I'd love to be able to get 5.1 digital output from my computer all the time, not just in AC-3 passthrough mode.

It's just a matter of figuring out how to take 3 S/PDIF signals and either turn them into AC-3 or some other 5.1 signal my reciever can use.

The most obvious solution is a DAC right before my reciever. At least then the signal will be digital until it is cleared from the EM interference of my computer.

Optical and coax carry the same signals (S/PDIF) just different mechanics. There are some companies (Alesis) that do multichannel (4-8?) over optical connectors, but this is something entirely different.

It sounds like the seperate DAC might work out well for you; this is actually what I am doing, a six channel DAC to do 5.1 decoding. The difference is that I am ripping the streams bodily from an off-the-shelf AC3 decoder, whereas you have them nicely packaged from your sound card.

There are some decent 6-channel DACs (single chip) or you could always go discrete for slightly more. This is MUCH easier than sythesizing AC3 in realtime, and will actually be higher quality.

It sounds to me like you already have exactly what you want available. Doing the AC-3 encoding will compress the digital streams, thus losing quality. By contrast, the 6-channel DAC sounds like the best option, since you'll get the full uncompromised quality (whatever that happens to be) of your SPDIF digital outs on all six channels.

DIY AC-3 encoding would be difficult at best, and is probably not something particularly useful to the DIYer. AC-3 decoding OTOH, is potentially very useful... especially if you have equipment which outputs a 5.1 encoded stream.

I have a soundblaster live! 5.1 card, and have wondered about this myself. There are a few pure software dolby digital(AC3) encoders available. http://www.soundmusic.com/sekd/ac3encoder.htm)
If something like this could be implemented into the SB live! drivers, then the 6 channel output that would normally be sent out the 3 stereo 1/8" jacks could be send out digitally as an AC3 stream in the same fashion that it is passed-through from DVD. The only question would be if your computer had enough power to encode it in real-time..